Hello all,
One of my ham friends recently found that his 40 meter dipole antenna became
detuned when he rotated his four element 20 meter Yagi antenna. The Yagi is
mounted atop a 20 meter tower and the dipole was mounted in Vee-fashion from
an extension tube at the top of the tower. At some beam headings the dipole
was quite worthless, acting as a cloud burner.
My friend got a very bright idea - using Eznec he checked the resonance
frequency of the boom of the 20 m Yagi, seen as a short dipole with
capacitive hats consisting of the reflector and the last director - and
bingo: the boom seems to have a low-Q resonance just above 7 MHz (estimated
impedance at 7 MHz: around 25 - j25 ohms or so).
Accordingly my friend tried to make the best of the situation: he mounted a
gamma match in line with the boom to feed it on 40 meters and was
successful, obtaining a good match to 50 ohms; now he has an excellent
rotary dipole on 40 meters and a good beam on 20 meters in the same package
and is working South American and Carbbean stations on 40 SSB like crazy
... The dipole feed arrangement for the boom is not interfering at all with
the 20 m Yagi function (the radiation modes are orthogonal).
Maybe my friend?s experience can inspire some of those people who observe
odd interactions between their beam antenna and other antennas - a long
boom Yagi for ten meters could have a nice boom resonance near 7 (or 10
MHz)...
73 Jan/SM0AQW
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